The present invention relates to a cryogenic rectification method and apparatus for separating a mixture into lower and higher volatility components with a reduced concentration of impurities in the lower volatility component. More particularly the present invention relates to such a method and apparatus as applied to the separation of air to produce a pumped liquid oxygen product having a reduced concentration of such heavy impurities as carbon dioxide and flammable hydrocarbons.
Mixtures are separated into their higher and lower volatility components by cryogenic rectification which is generally carried out in rectification columns having trays or packings. The separation is characterized by a countercurrent vapor-liquid contact of a descending liquid phase with an ascending vapor phase on the trays or within the packing. The descending liquid phase becomes ever more concentrated in the lower volatility components as it descends within the rectification column and the ascending vapor phase becomes ever more concentrated with the higher volatility components as it ascends within the rectification column. Heavy impurities concentrate within the descending liquid phase. In case of cryogenic air separations, heavy impurities such as carbon dioxide can present problems in carrying out the separation in the first instance.
As an example, in cryogenic air separation plants that produce gaseous oxygen at a delivery pressure by vaporizing pumped liquid oxygen in a main heat exchanger, heavy impurities such as carbon dioxide and hydrocarbons can exceed their solubility limit in the liquid oxygen as it vaporizes. As a result, carbon dioxide contained within the liquid oxygen can solidify to thereby plug up the heat exchange passageways within the main heat exchanger and hydrocarbons such as acetylene can come out of solution to present a safety hazard.
Generally speaking in case of liquid oxygen production, the heavy impurities are removed from the incoming air by an adsorptive prepurification unit. Some impurities, however, remain, and as a result, heavy impurities will concentrate within the lower volatility component of air, namely oxygen.
As will be discussed, the present invention provides a method to increase heavy impurity removal at the front end of the plant so that the volatile component to be separated has a reduced concentration of the heavy impurities.